Capacitor transducer

ABSTRACT

A capacitive transducer in which a pair of exciter electrodes connected to an AC source are disposed in common planes to present a gap. A pick-off electrode is disposed in another plane and extends on either side of the gap. A conductive screen is interposed between the exciter electrodes and the pick-off electrode and is connected to ground potential so as to block capacitive coupling between the exciter electrodes and the pickoff electrode except as between discrete areas of the exciter electrodes and an opposite area of the pick-off electrode exposed or coupled through an aperture in the screen.

Norrie CAPACITOR TRANSDUCER [75] Inventor: Georgeogilvie Norrie,Bromham,

England [731 Assignee= y ndis T99 ompavyzwaiqeh tqtfl [22] Filed: Feb.17, 1972 [21] Appl. No.: 227,156

Related US. Application Data [63] Continuation-impart of Ser. No. 7,187,Jan. 30,

Carter; 323/93 X Jan. 8, 1974 3,207,964 9/1965 Woods et a1 318/662 X3,213,360 10/1965 Cook et a1 324/61 P 3,372,333 3/1968 Esch 323/93 X3,543,259 11/1970 Klyge 340/200 3,570,003 3/1971 West i 323/93 3,312,8924/1967 Parnes 323/93 Primary Examiner-18. D. Pellinen Attorney-SpencerT. Smith et a1.

[ ABSTRACT A capacitive transducer in which a pair of exciter electrodesconnected to an AC source are disposed in common planes to present agap. A pick-off electrode is disposed in anotherplane and extends oneither side of the gap. A conductive screen is interposed between theexciter electrodes and the pick-off electrode and is connected to groundpotential so as to block capacitive coupling between the exciterelectrodes and the pick-off electrode except as between discrete areasof the exciter electrodes and an opposite area of the pick-off electrodeexposed or coupled through an aperture in the screen.

19 Claims, 11 Drawing Figures F f v SI 4 PAIENTEUJAN 14 3,784,897

sum 1 or {1 19 f ,5 @E E M NT [3 LE E 747 INVENTUR GEURGE u. NURRHE 5859 \QTTQ ml EYS PATENTEDJAN 8M4 3.784.897

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SHKU 3 UP 4 /NVENTOR GEORGE 0. NDRR1 E T DRNEYS CAPACITOR. TRANSDUCERCROSS REFERENCE TO RELATED APPLICATION This a licaiiiiii isa""c6iitiiiii5tl6ii-inan of m eopending application Ser. No. 7,187,filed Jan. 30, 1970, and now abandoned.

BACKGROUND OF THE INVENTION Capacitance devices are known in which thecapacitive coupling between a pick-off electrode arrangement and a pairof exciter electrodes is varied such that a differential effect isattained. In such devices, however, the effects of stray capacitancebetween the exciter electrodes and the pick-off electrode may besignificant, giving rise to inaccuracies, non-linearities or the likewhich may affect the usefulness of the device particularly inapplications such as for measuring relative movements between machineparts. For example, it may be desirable to utilize a differentialcapacitance device to provide accurate and precise electrical signaloutputs in response to minute feeding movements of a machine toolthereby to provide the basis for accurate and precise programming oftool movements so that sequentially produced parts each may be held toclose tolerances.

BRIEF SUMMARY OF THE INVENTION In a preferred embodiment of .theinvention, an electrically conductive screening plate is usedelectrically to isolate or decouple a pair of exciter electrodes, on theone hand, and a pick-off electrode on the other hand, except as betweendiscrete areas of the exciter electrodes and an opposed area of thepick-off electrode. For this purpose, the screening plate is connectedto ground potential and is interposed between the exciter electrodes andthe pick-off electrode to mask the same from each other except throughor across the dielectric region, in any operative position of the parts.

BRIFF DESCRIPTION OF THE DRAWING FIGURES FIG. I is a diagrammatic viewofa simple form of differential capacitor embodying the invention coupledto a machine tool element;

FIG. 2 is a similar diagrammatic view of a multicycle differentialcapacitor having provision of calibration;

FIG. 3 is a diagrammatic view similar to FIG. 2 showing a differentmethod of calibration, and an ambiguity resolver;

FIG. 4 is an exploded diagrammatic view of a differential capacitorsimilar to FIG. 2-except that it is arranged for rotary rather thanlinear movement;

FIG. 5 is a diagrammatic view of a modification of the capacitors ofFIGS. 2 and 3;

FIG. 6 is an exploded diagrammatic view of a different kind ofcapacitor;

FIG. 7 shows the electrodes ofthe capacitor of FIG. 6 developed into aplane;

FIG. 8 is a diagrammatic view of parts cut away of a linear differentialcapacitor embodying the invention;

FIG. 9 is a diagrammatic showing of still a different form of capacitorfor detection of linear movement and in which a stationary pick-offmember is used;

DETAILED DESCRIPTION OF THE INVENTION In general, differentialcapacitors have a pair of plates which are excited from the two ends ofan AC supply, a movable member which may be described as aninterpolator, and a pick-off which gives an electrical signal whichchanges with the position of the interpolator in relation to the excitedplates. In many forms of differential capacitor the pick-off is also theinterpolator.

In the differential capacitor shown in FIG. 1 exciter plates 11 and 12are spacedapart by a gap 13 which is oblique to the directionof'movement illustrated by arrow 14 of an interpolator 15, which alsoacts as a pick-off. The plates 11 and 12 are energized from the two endsrespectively of a reference supply comprising a secondary winding 16 ofa transformer 17 and it can be seen that the coupling between theinterpolator 15 and each of the plates 11 and 12 depends upon the linearposition of the interpolator in the direction of arrow 14 by virtue ofthe changing overlapping plate area. The voltage of this interpolator 15is a measure of its position and a signal can be obtained from aterminal (not illustrated) connected to it. The interpolator 15 may besurrounded by, but spaced from a shield 20 for minimizing distortionatthe ends of the range.

It will be appreciated that the device need not be much longer than thedesired rangeof movement of the interpolator 15, it being only necessaryto provide provision for suitable mechanical bearings and appropriateshielding to define the electric field and prevent distortion by themounting components, for exmple bearings. For example, if the range ofmovement is 5 inches, the device could be about 6 inches long. Also theangle of the gap 13 can be chosen in accordance with desired rate ofchange of the pick-off signal.

A simple means for calibration can be provided by a slope correctingplate 18 extending along the length of the capacitor and separated fromthe plate 11 by a gap 19. This plate 18 is energized from a separatesecondary winding 21 of the transformer 17. The coupling between theinterpolator 15 and the correcting plate 18 can be adjusted by screwingin or out of conducting mechanical pins 22 toward plate 18 carried bythe interpolator 15 to adjust the value of acorrecting signal. If asimilar arrangement is used at the other side of the capacitor, that isto say a slope correcting plate 23 spaced from the plate 12 by a gap 24calibration can be effected without moving the electrical center of thecapacitor. Further, the plate 23 maybe energized from a secondarywinding (not shown) of the transformer 17, and conducting mechanicalpins 25 may be provided to adjust the coupling between a plate 23 andthe interpolator 15. Similarly, the coupling capacitance between thepick-off and each reference electrode can be adjusted to a standardvalue which can be of advantage when operating such transducers in acaliper. mode.

Of course with adjustment, the mechanical pins 22 or the energizedcorrecting plate 18, could be used by itself, rather than together asshown in FIG. 1.

, The interpolator 15, as illustrateddiagrammatically, is connected to amachine tool element 26, which may be a carriage for supporting aworkpiece (not illustrated) or a grinding wheel not illustrated) or thelike by mechanical means diagrammatically illustrated as dashed line 27.

Alternatively, the machine tool element 26 may be mechanically coupledto the exciter electrode assembly 11, 12 etc. while the interpolator 15is held in a fixed position. Further, in this case, it may beadvantageous to arrange for a multiplicity of pick-off elements 15 to bemounted on shield and for a switch system to select the appropriateelement 15 depending on the position of the moving exciter electrodeassembly ll, 12, etc. as it moves past the pick-off elements 15.

The arrangement of FIG. 2' provides for a much greater linear movement,for example, 16 inches, and in this case the two exciter plates are ofcomplementary saw-tooth shape as shown at 28 and 29 separated by a gap30.

The interpolator 15A comprises a plate 31 genrally similar to theinterpolator 15 of FIG. 1 in conjunction with a second similar plate 32,by a non-conductive mounting member diagrammatically illustrated at 33,to move with it and spaced so that the plate 31 will be moving over thecentral part of one inclined gap or ramp as shown at 34 while the otherplate 32 is moving over the peak 35 joining two ramps. Then the secondplate 32 will be moving over the central part of the next ramp 36 whilethe first plate 31 is moving over the peak 35. A simple electrically orcam-operated switch 37 is arranged to switch a signal from either lead38 or lead 39 connected respectively to the interpolator plates 31 and32 to an output terminal 40 depending on which plate is over the centralpartof a ramp in the position of the interpolator concerned. The plates28 and 29 are energized from an AC source by a pair of leads 4].

As with the emboidment of FIG. 1, calibration corrections can be made byuse of correcting plates 42 and 42A along the edges of the capacitor,and plate 42 can be arranged to be connected to a selected tapping, forexample 43, on a secondary winding 44 of a transformer 45 by operationof a selector switch 46. The switch 46 can be selected by an appropriatecam (not illustrated) or by an electrical logic circuit (notillustrated) and the tap that will be selected will depend on thecorrection required for the particular ramp 34 or 36 over which theinterpolator plate 31 or 32 is moving when the cam or logic-circuitcontrolled switch is operated. There will be a similar arrangement forthe correcting plate 42A which may include an additionalsecondarywinding 47 and a switch 48.. It will be appreciated, that theinterpolator 15A may be mechanically coupled to a machine tool elementin a similar manner to interpolator 15 as shown in FIG. 1.

The arrangement of FIG. 3 shows how calibration correction can beaffected in a different way by having a plurality of correcting plates49, 49A, etc. one for each of the ramps50, 51, etc. Each will beconnected to an appropriate tap on a secondary winding (not illustrated)or to separate taps on secondary windings such as 44 and 47 asillustratedin FIG.2. FIG. 3 also shows how ambiguity between the variousramps can be avoided by additional elongated wedge like plates 52 and 53with its individual pick-off plate 54 ganged, by a non-conductivemounting member diagrammatically illustrated at 57, to move with theplates 58and 59 and connected to a circuit (not illustrated) forindicating 4 the number of complete ramps covered or cycles moved, alongthe length of the capacitor. Plates 58 and 59 are ganged together by themember 57 and comprise an interpolator 158. The capacitor illustrated inFIG. 3 includes two exciter plates '55 and 56 separated by a saw-toothgap 60. The two exciter plates are energized from an AC source via apair of leads 61.

The interpolator 158 may be mechanically coupled to a machine toolelement in a similar manner as interpolator 15 as shown in FIG. 1.

FIG. 4 shows how the capacitor can be arranged to detect angular orrotary movements. In this case the exciter plates 62 and 63 havecomplementary saw-tooth edges separated by a gap 64 and disposed aroundthe periphery of a cylindrical shell 77; They are energized from asecondary winding 65 of a transformer 66, and the interpolator 15Cincludes plates 67 and 67A ar ranged on a rotor 68 to turn about theaxis 69 of the device. It is believed that the analogy to the embodimentof FIG.- 2 will be so clear that further description is not necessaryand of course correcting plates 70 and 71' can be added in the same wayas described. It is preferred that the angle between successive cyclesof the saw-tooth will represent 60 angular degreesas indicated at 72 toproduce 6 cycles'in one revolution. While a pair of pick-off plates 67and 67A, one for each of adjacent ramps are shown, it may beadvantageous to provide two pairs of interpolator plates l apart inorder to provide corrections for slight eccentricity or other mechanicalinaccuracy as illustrated by additional interpolator plates 73 and 74.The capacitor may include the calibration correction plates 70 and 71separated from the exciter plates 62 and 63 by gaps 75 and 76 andarranged on the ends of the cylindrical shell 77. Of course, separatecalibration correctionelectrodes may be provided for each ramp asillustrated in FIG. 3,

FIG. 4 may be arranged so that interpolator 15C is mechanically coupledto a machine tool element or the like and thereby be suitable to providesignals indicative of the angular displacementor motion of the machinetool element or the like.

FIG. 5 shows an alternative arrangement with multiramp tracks butwithout having an interpolator comprising two pick-off plates spacedapart along the length of a single track.

It will be seen that there are three tracks having respective exciterplates '78 and 79,80 and 81, and 82 and 83 separated respectively bysaw-tooth gaps 84, 85 and 86 and energized from secondary windings 87,88 and 89 of a transformer (partially illustrated). The first two trackshave gaps 84 and 85 which are of the same pitch, but are phaseddisplacedby'a quarter of a pitch.

The interpolator 15D comprises pick-off electrodes 90,

trode 91 is moving over a peak-of its track and vice versa, so that byappropriate mechanical or electrical.

switching (not illustrated) or by appropriate alternate energization ofthe tracks a continuous signal can be derived from the two pick-offelectrodes 90 and 91.

The third pick-off electrode 92 is merely for avoiding ambiguities andits track has the gap 86 which has a greater pitch than the gaps 84 and85 associated with the first two tracks.

In an alternative arrangement the shield 93 may take the form of anapertured screen which does not have the pick-off electrodes 90,91 and92 in its apertures, and is arranged to move along the tracks and ispositioned between a set of fixed pick-off electrodes in spacedface-to-face relation with the tracks. The aperture in a screen, in thisalternative arrangement, varies the coupling between each pick-offelectrode and the exciter electrodes of respective tracks as it movesalong the tracks. Inthis instance, outlet leads are connected to fixedpick-off electrodes rather than to moving electrodes.

In the capacitor of FIG. 6 the interpolator D includes pick-offelectrode assembly 96 arranged to move axially within a cylindricalshell 97. The pick-off electrode comprises an external cylindricalconducting surface 98 on a cylindrical member 99. The exciter electrodearrangement consists of conducting surfaces 100 and 101 withcomplementary saw-tooth-like shapes separated by gaps 102 as can be seenbest from the developed view in FIG 7. As the pick-off 96 moves over therange of measurement 103 (FIG. 7), the coupling with one of the surfaces100, 101 will increase and the coupling with the other of the surfaces100, 101 will decrease. The division of the exciter plates into twoteeth each allows some compensation for slight eccentricity inassembling the pick-off electrode 96 with the cylindrical shell 97.Electrical connections to the conducting surfaces 100 and 101 areprovided respectively by a wiring-arrangement 104 and 105.

It will be appreciated that the interpolator 15D may be mechanicallycoupled to a machine tool element 106 or the like as diagrammaticallyillustrated by dashed line 107.

The differential capacitor of FIG. 8 has a pair of exciter capacitorplates 108 and 109 each insulated from and laced around the innercylindrical surface of a fixed cylindrical housing 110 the two of thembeing connected across the ends of a secondary winding 111 constitutinga reference AC supply and forming part of a transformer 112. There is ashort annular gap 113 between the plates 108 and 109.

A pick-off signal representative of the linear position of aninterpolator 15E shown as connected to a stylus 114 is derived from afixed pick-off electrode 115 in the form of an elongated cylindercoaxial with the plates 108 and 109 and extending throughout the majorpart of the combined lengths of the plates being symmetrically disposedin relation to the plates. The pick off electrode 115 is connected to apick-off terminal or lead 115A.

The interpolator 15E whose movement is to be detected comprises a gap116 between the components of an electrically conducting cylindricalshell screen consisting of two conducting shells 117 and 118 separatedby the gap 116 but held together by a web 119 of uniform width, andelectrically connected together. They are disposed coaxially with therest of the capacitor surrounding the pick-off electrode 115 and withinthe plates 108 and 109. The gap 116 between the facing edges of theconducting shells 117 and 118' of the screen is substantially wider thanthe gap 113 but less than the length of the pick-off electrode 115. Thescreen consisting of shells 117, 118 and web 119 is earthed.

It will be appreciatpd that the position of a gap 116 in a screenbetween either exciter plate 108 or plate 109 and the pick-off electrodewill determine the coupling between the plates and the pick-offelectrode 1 15 so that as linear movement takes place of the stylus 114with the sreen, the coupling of the pick-off electrode 115 to one of theexciter plates 108 and 109 will increase while the coupling to the otherwill decrease and accordingly a differential output signal can beobtained from the terminal 115A representative of the linear position ofthe stylus 114.

The arrangement has the advantage that both the exciting referencesupply and the pick-off signal are connected to fixed terminals whilethe moving electrical component only has to be grounded through somesort of flexible connection so that slip rings are not necessary for thesignal output. l

It will be appreciated that rather than being connected to a stylus orin addition to the stylus, the interpolater 15E may be-connected orcoupled to a machine tool element or the like so thatthe output fromterminal 115A may represent the position or movement of the machine toolelement with respect to a workpiece or another part of a machine tool. I

A preferred embodiment of the invention according to the principlesshown in FIG. 8 is illustrated in FIG. 11 wherein a housing of thetransducer corresponds to the housing 110 of FIG. 8; exciter electrodes161 and 162 correspond to the plates 108 and 109; the screening meanscomprises portions 163, 164 and an interconnecting web 165 correspondsto the shells 117, 118 and a web 119; the pick-off electrode 166corresponds to the electrode 115; and the stylus 167 corresponds to thestylus 114. I

The pick-off electrode 166 is carried by a support assembly whichincludes a tubular member 168 having an externally threaded reduced endportion 169 and an internally threaded counterbore 170 at its other end.A spindle or rod member 171 is provided with external threads and isengaged at one end in the counterbore 170 whereas its opposite end is inthe form of an enlarged head 172 provided with a threaded recessreceiving the screw fastener 173. The pick-off electrode 166 iselectrically isolated from the spindle 171 by means of bevelled washers174 and 175 of a nonconductive material. The washers 174-and 175 locatethe pick-off electrode 166 radially when the member 168 and the spindle171 are threaded together.

The housing 160 receives a cylindrical support plug 176 having a centralopening through which the threaded end portion 169 projects and a nut177 firmly attaches the entire support assembly within the housing 160.At its opposite end, the housing 160 is internally threaded and receivesa threaded anchor ring 178 which supports the opposite endof the spindle171 through the medium of a strap or guide post 179 which is heldagainst the head end of the spindle 171 by means of the aforementionedfastener 173. Screw fasteners 180 secure the opposite ends ,of the guidepost 179 to the ring 178.

The spindle 171 is provided with a longitudinal groove 181 of sufficientdepth more or less and in alignment with a bore 182 of the tubularmember 168 and thereby to provide a continuous passage through which acondutor 183 extends for connection to the pick-off electrode 166. v

The exciter electrodes 161 and 162 present a gap 184 and are locatedwithin the housing 160 by means of respective bushings 185 and 186ofnon-conductive material. The bushings 185 and 186 are of substantiallyequal lengths and the pick-off electrode 166 is essentially centeredlongitudinally with respect to them and may extend somewhat beyond theopposite ends thereof, as shown. The pick-off electrode 166 is of coursesubstantially concentric with respect to the exciter electrodes 161 and162.

The screening means includesthe two electrically conductive portions 163and 164 which define an aperture or a dielectric region therebetweenthrough or across which discrete areas of the two exciter electrodes 161and 162 provide a capacitive coupling with the opposed area of thepick-off electrode 166. The portions 163 and 164 extend well beyond theopposite ends of the pick-off electrode 166 so as always to mask theexciter electrodes 161 and 162 from the pick-off electrode 166 exceptfor the capacitive coupling allowed as aforesaid. Thus, stray capacitivecoupling between the electrodes 161 and 162 'or connecting leads to theexciter electrodes 161 and 162 and to the pickoff electrode 166 isprevented'such as might detrimentally affect the pick-off signalproduced by the differential capacitance effect as the screening meansis shifted longitudinally during measurements. Thus, it will be seenthat the coupling path is a clearly defined area dicapacitor to operatewith extreme precision, without the need to provide a direct electricconnection to obtain an output signal which is a function of theposition of the relatively moving member.

To this end, the dielectric region presented between the portions 163and 164 is positioned always to bridge across the gap 184, andmechanical means are provided to limit the range of movement of thescreening means with respect to the electrodes 161 and 162. The guidepost 179 may function to limit the axial move ment of the screeningmeans by virtue of the locations and the finite lengths of slots 187 inthe screen portion.-

164 through which the guide post 179 projects. The slots 187 may be ofonly slightly greater width than the guide post 179 so as to preventrelative rotation of the screening means and of the stylus 167.

To locate the screen portions 163 and 164 concentrically with respect tothe pick-off electrode 166 and to allow for axial movement of the screenportions 163 and 164, bushings 188 and 189 are provided on the tubularmember 168 and head 172 of the spindle 171, respectively. It will beunderstood that the screen portions 163 and 164 are connected to groundpotential at the outer end of the bushing 188 or the bushing 189 by acoil spring (not shown) or by other electrical conducting means. Theexciter electrodes 161 and 162 are connected in circuit as described inconjunction with FIG. 8. It will also be understood that the screeningmeans may take the form of a tubular dielectric member having theelectrically conductive portions deposited thereon to define an uncoateddielectric region therebetween through which the capacitive couplingtakes place. Further, it will be appreciated that the pick-off electrodeassembly 96 of FIG. 6 may be relarectly through a single dielectricregion to' permit the tively fixed and a movable screen, as in FIGS. 8and 1 1, utilized therewith; 1

FIGF9 illustrates a differential capacitor similar, in some respects, tothat shown in FIG. 1 and shows how the invention can be applied to adevice for linear movement in which the exciter plates and 121 and apick-off electrode 129 are stationary. In this case, two exciter plates120 and 121 are connected in series across a secondary winding 122 of atransformer 123, and have complementary edges separated by a gap 124which is oblique to the direction of movement indicated by arrow 128.

panama exciter plates 121) and 121 is spaced fret ible connection meansor other suitable arrangement.

Movement of the aperture 127 as indicated by the: I

arrow 128 will increase the coupling between one of the plates 120 and121 and the pick-off electrode 129 and at the same time decrease thecoupling between the other plate and theelectrode 129 so that thedifferential output signal can be derived from a terminal (notillustrated) connected tothe electrode 129.

It will be understood thatthe transducer of FIG. 9, as is the case withthose shown in FIGS. 8 and 11, is dimensionedsuch that the groundedscreen 126 is interposed between or masks the exciter electrodes 120 and121 from the pick-off electrode 129 except as between those discreteareas of the exciter electrodes which are coupled through the aperture127 to the opposed area of the pick-off electrode 129. By inspection, itwill be seen that a similar situation pre ils for FIG. 10.

The shape of the output signal from the electrode 129 may be improved,so far as accuracy is concerned, by providing that exciter plates 120and 121 be additionally energized respectively from secondary wind-.ings 122A and 1228 of a transformer 123A, the secondary wingings 122Aand 122B beng connected in series with the secondary winding 122. thetransformers 123 and 1235 are connected to the same source.

It will be appreciated that thefcapacitor as illustrated in FIG. 9 maybe advantageously provided with a shield and/or slope correctingconductive screws in. a similar FIG. 10 illustrates a capacitor in whicha three track arrangement is provided. An interpolator 156 comprises anelectrically conductive cylinder 130 having three axially alignedapertures 131, 132 and 133 therein. The interpolator 15G is operativelyarranged to rotate about an axis 134 as. illustrated by the arrow 135..Positioned about and on the outside'of the interpolator 150 are threetracks each of which includes two exciter plates of complementarysaw-tooth shape and illustrated as exciter plates 136 and 137, 138 and139, and 140 and 141 separated respectively by gaps 142, 143 and 144.The first two tracks have the gaps 142 and 143 which are of the samepitch, but are phase displaced by, for example, a quarter of a pitch.The pitch of the gap 144 is a greater pitch than the pitch of the gaps142 and 143, and is for the purpose of avoiding ambiguities. Theelectrodes 136 and 137, 138 and 139, and 140 and 141 are respectivelyenergized from secondary windings 145, 146, and 147 of a transformer148.

Positioned within 1 56 is a stationary cylinder 149 which has positionedon the outer surface.

thereof a plurality of electrodes 150, 151 and 152. The electrode 150 isassociated with the aperture 131 of the interpolator 156, the electrode151 is associated with the aperture 132 and the electrode 152 isassociated with the aperture 133 of the interpolator 156. It can be seenthat the aperture 131 will be moving over a ramp of its track as theaperture 133 is moving over a peak of its track and vice versa, so thatby appropriate mechanical or electrical switching (not illustrated) orby appropriate alternate energization of the exciter electrodes 136 and137 and 138 and 139 of the tracks a continuous signal may be derivedfrom two of the pickoff electrodes 150 and 151. The third pick-offelectrode 152 adjacent the moving aperture 133 will provide a separatesignal for avoiding ambiguities for its track and is associated with thegap 144 which has a greater pitch than the gaps 142 and 143. It will beappreciated that the tracks may be separated from one another by earthed(grounded) electrodes and further that earthed (grounded) electrodes maybe provided ajacent the endmost exciter electrodes 136 and 141 in amanner similar electrically to the capacitor illustrated in F IG. 5. Anappropriate electrically operated or camoperated switching means (notillustrated) can be arranged to select an output from one each of theelectrodes 150 or 151, and/or 152 depending on the arouate position ofthe apertures 131, 132 and 133 of the interpolator 156. The electricallyconductive cylinder 130 is earthed (grounded).

The interpolator 156 may be coupled to a machine While the arrangementsshown and described above represent illustrative and preferred forms ofthe invention, it will be readily apparent to one skilled in the artthat variations may be made in such arrangements without departing fromthe scope. and spirit of the invention as defined in the appendedclaims. For example, the capacitor illustrated as cylindrical may bealternatively constructed as flat capacitor and those illustrated asflat capacitors may be constructed as cylindrical capacitors.

Another possible modification is that additional secondary windings maybe connected in series with secondary winding 111, shown in FIG. 8 forfeeding a corrective voltage to either or both exciter electrodes 108and 109 to improve the accuracy or modify the output from pick-offelectrode 115; the arrangement is similar to the corrective arrangementillustrated in FIG 9.

I claim:

l. lnia capacitive transducer, the combination comprising:

first and second exciter electrodes fixedly located in a common planeand having adjacent spaced-apart edges defining an electricaldiscontinuity therebetween, I

a pick-off electrode fixedly located adjacent said first and secondexciter electrodes in spaced relation from the common plan thereof andspanning the electrical discontinuity therebetween,

means for connecting said first and second exciter electrodes to sourcesof different potential to induce a signal on said pick-off electrodeindicative of the portions of said first and second electrodes spannedby said pick-off electrode,

a shield located between said pick-off electrode and said first andsecond exciter elecrodes and connected to a ground potential to preventa signal from being induced on said pick-off electrode,

said shield having a dielectric aperture therein bridging the electricaldiscontinuity for permitting portions of said first and second exciterelectrodes adjacent the electrical discontinuity to induce a signal onsaid pick-off electrode,

said shield being movable relative to said first and second exciterelectrodes about the electrical discontinuity to vary the portions ofsaid first and second exciter elements inducing a signal on said pickoffelectrode to produce a signal responsive to the position of said movableshield, and

said shield being dimensioned to prevent stray capacitive couplingbetween said pickoff electrode and said first and second exciterelectrodes at all positions of movement of said shield.

2. A capacitive transducer as defined in claim 1 wherein said exciterelectrodes each are of tubular form and are disposed in end-to-endspaced relation to define said electrical discontinuity and wherein saidshield has electrically conductive portions of tubular configurationlocated in end-to-end spaced relation to define said dielectric region.

3. A capacitive transducer as defined in claim 2 including a tubularhousing within which said exciter electrodes are fixed, said pick-offelectrode being in the form of an elongated sleeve concentricallydisposed withrespect to said exciter electrodes, and said shield beingof a length to completely mask said exciter electrodes from saidpick-off electrode except through said dielectric region.

4. A capacitive transducer asdefined in claim 3 including a supportassembly located axially within said housing and concentrically locatingsaid pick-off electrode thcrewithin, and means for slidably supportingsaid shield on said support assembly.

5. A capacitive transducer as defined in claim 4 including a guidemember disposed diametrically within said housing, said shield havingslots formed therein for slidably receiving said guide member to preventrotation of said shield means relative to said housing.

6. A capacitive transducer as defined in claim 1 wherein said pick-offelectrode, said exciter electrodes and said shield are in the form offlat plates.

7. A capacitive transducer as defined in claim 1 wherein said exciterelectrodes are eachof annular form and having zig-zag edge portionsdisposed in spaced relation to define said electrical discontinuity,

' said pick-off electrode being concentrically disposed with respect tosaid exciter electrodes, and said shield being rotatable between saidexciter and pick-off electrodes.

'8. A capacitive transducer as defined in claim '1 wherein theelectrical discontinuity defined by the adjacent edges of said first andsecond exciter electrodes is a straight gap of uniform width.

9. A capacitive transducer as defined in claim 1 wherein the electricaldiscontinuity defined by the adjacent edges of said first and secondexciter electrodes is a saw-tooth gap of uniform width.

10. A capacitive transducer as claimed in claim 1 including acompensatory .electrodemeans positioned near one of said exciterelectrodes and separated therefrom by a gap, said compensatory electrodemeans being coupled to said pick-off electrode.

11. A capacitive trasducer as claimed in claim including a secondcompensatory electrode positioned near the other of said exciterelectrodes and separated therefrom by a gap, said second compensatoryelectrode being coupled to said pick-off electrode.

12. A capacitive transducer as claimed in claim 11 including means forvarying the coupling between at least one of said compensatoryelectrodes and said pick-off electrode.

13. A capacitive transducer as defined in claim 1 in 15. A capacitivetransducer as claimed in claim 14 including a compensatory elecrodemeans positioned is a zig-zag gap along the path of relative movementbetween said shield and said exciter electrodes, and further including asecond pick-off electrode and a second pair of exciter electrodesassociated therewith, said second pair of exciter electrodes havingadjacent edges defining a second electrical discontinuity 'therebetweenextending at an acute angle of said path of relative movement, and saidshield defining a second dielectric aperture therein bridging the secondelectrical discontinuity between said second pair of exciter electrodes.

18. A differential capacitor as claimed in claim 17 in- 7 cluding acompensatory electrode means positioned riear one of said exciterelectrodes and separated therefrom by a gap, said compensatory electrodemeans being coupled to said pick-off electrode. A

19. A differential capacitor as claimed in claim 18 including a secondcompensatory electrode positioned near the other of said exciterelectrodes and separated therefrom by a gap, said second compensatoryelectrode being coupled to said pick-off electrode.

Dated January 8, 1974 Patent No. 43897' Inventor(s) GQ-rge -OgilvieNorrie It is certified that error appears in the above-identified patentand that said Letters Patent are hareby co r rected as shown below;

Column 5,, li n e +4, "laced" should read--placed-.

- In The Claims Claims 16*, 18 and 19 line 1, "differential capacitor"shquld readcapacitive cJ:ansducer---'-.v

Signed and seal d this 16th day of July 1974.

(S AL) 'Attest:'

MCCOY M. GIBSON, JR. 0. MARSI ALL DANN Attesting Officer Commissmner ofPatents IIINITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 45 7 v Dated January 8, 1974 Inventofl) Gorg Ogilvie Norrie- It iscertified that error appears it the above-identified patent and thatsaid Letters Patent are hexehy co r rected as shovm below;

Column 5,, line 111+, "laced" should read--placed--.

In The Claims I I Claims 16", 18 and 19, line 1, "differentialcapacitor" shquLd re.ad- -c;apa citive transducer--'-.

Signed and seal d this 16th day of Jul 1971.

(SEAL) I Attest:

G. MARSHALL DANN MCCOY M. GIBSON, JR. Attesting Officer r COmIHlSSlQIIGIof Patents

1. In a capacitive transducer, the combination comprising: first andsecond exciter electrodes fixedly located in a common plane and havingadjacent spaced-apart edges defining an electrical discontinuitytherebetween, a pick-off electrode fixedly located adjacent said firstand second exciter electrodes in spaced relation from the common planthereof and spanning the electrical discontinuity therebetween, meansfor connecting said first and second exciter electrodes to sources ofdifferent potential to induce a signal on said pickoff electrodeindicative of the portions of said first and second electrodes spannedby said pick-off electrode, a shield located between said pick-offelectrode and said first and second exciter elecrodes and connected to aground potential to prevent a signal from being induced on said pickoffelectrode, said shield having a dielectric aperture therein bridging theelectrical discontinuity for permitting portions of said first andsecond exciter electrodes adjacent the electrical discontinuity toinduce a signal on said pick-off electrode, said shield being movablerelative to said first and second exciter electrodes about theelectrical discontinuity to vary the portions of said first and secondexciter elements inducing a signal on said pick-off electrode to producea signal responsive to the position of said movable shield, and saidshield being dimensioned to prevent stray capacitive coupling betweensaid pick-off electrode and said first and second exciter electrodes atall positions of movement of said shield.
 2. A capacitive transducer asdefined in claim 1 wherein said exciter electrodes each are of tubularform and are disposed in end-to-end spaced relation to define saidelectrical discontinuity and wherein said shield has electricallyconductive portions of tubular configuration located in end-to-endspaced relation to define said dielectric region.
 3. A capacitivetransducer as defined in claim 2 including a tubular housing withinwhich said exciter electrodes are fixed, said pick-off electrode beingin the form of an elongated sleeve concentrically disposed with respectto said exciter electrodes, and said shield being of a length tocompletely mask said exciter electrodes from said pick-off electrodeexcept through said dielectric region.
 4. A capacitive transducer asdefined in claim 3 including a support assembly located axially withinsaid housing and concentrically locating said pick-off electrodetherewithin, and means for slidably supporting said shield on saidsupport assembly.
 5. A capacitive transducer as defined in claim 4including a guide member disposed diametrically within said housing,said shield having slots formed therein for slidably receiving saidguide member to prevent rotation of said shield means relative to saidhousing.
 6. A capacitive transducer as defined in claim 1 wherein saidpick-off electrode, said exciter electrodes and said shield are in theform of flat plates.
 7. A capacitive transducer as defined in claim 1wherein said exciter electrodes are each of annular form and havingzig-zag edge portions disposed in spaced relation to define saidelectrical discontinuity, said pick-off electrode being concentricallydisposed with respect to said exciter electrodes, and said shield beingrotatable between said exciter and pick-off electrodes.
 8. A capacitivetransducer as defined in claim 1 wherein the electrical discontinuitydefined by the adjacent edges of said first and second exciterelectrodes is a straight gap of uniform width.
 9. A capacitivetransducer as defined in claim 1 wherein the electrical discontinuitydefined by the adjacent edges of said first and second exciterelectrodes is a saw-tooth gap of uniform width.
 10. A capacitivetransducer as claimed in claim 1 including a compensatory electrodemeans positioned near one of said exciter electrodes and separatedtherefrom by a gap, said compensatory electrode means being coupled tosaid pick-off electrode.
 11. A capacitive trasducer as claimed in claim10 including a second compensatory electrode positioned near the otherof said exciter electrodes and separated therefrom by a gap, said secondcompensatory electrode being coupled to said pick-off electrode.
 12. Acapacitive transducer as claimed in claim 11 including means for varyingthe coupling between at least one of said compensatory electrodes andsaid pick-off electrode.
 13. A capacitive transducer as defined in claim1 including a second pair of exciter electrodes defining a secondelectrical discontinuity therebetween, a second pick-off electrode, andsaid shield having a second dielectric aperture therein bridging saidsecond electrical discontinuity.
 14. A capacitive transducer as definedin claim 13 wherein the electrical discontinuity defined by the adjacentedges of said first and second exciter electrodes is a saw-tooth gap ofuniform width.
 15. A capacitive transducer as claimed in claim 14including a compensatory elecrode means positioned near one of saidexciter electrodes and separated therefrom by a gap, said compensatoryelectrode means being coupled to said pickoff electrode.
 16. Adifferential capacitor as claimed in claim 15 including a secondcompensatory electrode positioned near the other of said exciterelectrodes and separated therefrom by a gap, said second compensatoryelectrode being coupled to said pick-off electrode.
 17. A capacitivetransducer as defined in claim 1 wherein the electrical discontinuitydefined by the adjacent edges of said first and second exciterelectrodes is a zig-zag gap along the path of relative movement betweensaid shield and said exciter electrodes, and further including a secondpick-off electrode and a second pair of exciter electrodes associatedtherewith, said second pair of exciter electrodes having adjacent edgesdefining a second electrical discontinuity therebetween extending at anacute angle of said path of relative movement, and said shield defininga second dielectric aperture therein bridging the second electricaldiscontinuity between said second pair of exciter electrodes.
 18. Adifferential capacitor as claimed in claim 17 including a compensatoryelectrode means positioned near one of said exciter electrodes andseparated therefrom by a gap, said compensatory electrode means beingcoupled to said pick-off electrode.
 19. A differential capacitor asclaimed in claim 18 including a second compensatory electrode positionednear the other of said exciter electrodes and separated therefrom by agap, said second compensatory electrode being coupled to said pick-offelectrode.